AGI I am not familiar with that phone nor I had heard of Lineage before.
Lineage OS is formerly Cyanogen Mod and is the largest custom Android ROM.
AGI Did you measure the device does not use PWM or are you relying on some website?
The Moto G4 Play does not have PWM per Notebookcheck.com's testing and from pencil/camera testing at home. It's possible it has super high PWM outside the range of the tests.
AGI Did the original OS (Android?) generate eyestrain instantaneously?
I never used it with the original, as I bought it just to put a custom ROM on and experiment and did so immediately.
AGI Is Lineage stable / flawless, like, camera and other functionalities all work the same as on OEM software?
It's as stable as any custom ROM. It depends on the device and maintainer. Usually everything works but bugs do happen from time to time. With most custom ROMs the phones are usually maintained by one volunteer, sometimes super popular phones have two or three people, and they can stop at any time leaving you high and dry...so it's a drawback to using them. Cameras are usually not great on custom ROM's because manufacturer post processing software is key to making images look good, and the camera software replacements used in custom ROMs aren't up to par. Depending on the device you will normally get mediocre to lower end results from the camera. A few devices like Pixels are able to use a port of Googe's camera software and still take good photos.